Androd Sample app showing black screen instead of camera

Running Samples from Android Studio ( Android Code Not Unity)

Tools - Vuforia 6.5

Devices - MotoCPlus (7.0) , MotoG5 Plus (7.0)  , LG Nexus 4 (6.0) 

Problem : Black Screen when launching any sample 

Here is How I m Building the sample :

1. open vuforia samples in android studio.

2. Add vuforia.jar , arm64-v8a and armeabi-v7a folders in libs folder

3. In build.gradle change dependency to  - compile fileTree(dir: 'libs', include: '*.jar')

4. Add development key in Vuforia.setInitParameters(mActivity, mVuforiaFlags, key);

5. After running the project , on launching app I see list of option , on clicking any option I found black screen instead of camera.

On Logging App :

I found app is not going after this line Vuforia.setInitParameters(mActivity, mVuforiaFlags, key); and it seems onPostExecute is not called or app hangs.

Solutions Tried :

1. Check any space in key

2. Thought in Marshmallow and Nouget , permission issue should be there and app was not asking for any permissions. So put the permission check code.

3. Checked on Multiple phones

Please Note : (This seems to be an issue as the app is not requesting any camera permissions)

Vuforia::init will now request the camera permissions via a system dialog on applications targeting Android M and above.

I request you to please help me out urgently. I last created an AR app 2 year back using vuforia and I didn't face any issue then.